Stellar Accretion Rate Research Articles

EVOLUTION OF VERY MASSIVE POPULATION III STARS WITH MASS ACCRETION FROM PRE-MAIN SEQUENCE TO COLLAPSE

We calculate the evolution of zero-metallicity Population III (Pop III) stars whose mass grows from the initial mass of ~1 M ☉ by accreting the surrounding gases. Our calculations cover whole evolutionary stages from the pre-main sequence, via various nuclear burning stages, through the final core-collapse or pair-creation instability phases. We adopt two different sets of stellar mass accretion rates as our fiducial models. One is derived from a cosmological simulation of the first generation (PopIII.1) stars, and the other is derived from a simulation of the second generation stars that are affected by radiation from PopIII.1 stars. The latter represents one case of PopIII.2 stars. We also adopt additional models that include radiative feedback effects. We show that the final mass of Pop III.1 stars can be as large as ~1000 M ☉, beyond the mass range (140-300 M ☉) for the pair-instability supernovae. Such massive stars undergo core-collapse to form intermediate-mass black holes, which may be the seeds for merger trees to supermassive black holes. On the other hand, Pop III.2 stars become less massive (40-60 M ☉), being in the mass range of ordinary iron core-collapse stars. Such stars explode and eject heavy elements to contribute to chemical enrichment of the early universe as observed in the abundance patterns of extremely metal-poor stars in the Galactic halo. In view of the large range of possible accretion rates, further studies are important to see if these fiducial models are actually the cases.

THE EFFECTS OF RADIATIVE TRANSFER ON LOW-MASS STAR FORMATION

Forming stars emit a substantial amount of radiation into their natal environment. We use ORION, an adaptive mesh refinement (AMR) three-dimensional gravito-radiation-hydrodynamics code, to simulate low-mass star formation in a turbulent molecular cloud. We compare the distribution of stellar masses, accretion rates, and temperatures in the cases with and without radiative transfer, and we demonstrate that radiative feedback has a profound effect on accretion, multiplicity, and mass by reducing the number of stars formed and the total rate at which gas turns into stars. We also show, that once star formation reaches a steady state, protostellar radiation is by far the dominant source of energy in the simulation, exceeding viscous dissipation and compressional heating by at least an order of magnitude. Calculations that omit radiative feedback from protstars significantly underestimate the gas temperature and the strength of this effect. Although heating from protostars is mainly confined to the protostellar cores, we find that it is sufficient to suppress disk fragmentation that would otherwise result in very low-mass companions or brown dwarfs. We demonstrate that the mean protostellar accretion rate increases with the final stellar mass so that the star formation time is only a weak function of mass.

ACCRETION DISKS AROUND MASSIVE STARS: HYDRODYNAMIC STRUCTURE, STABILITY, AND DUST SUBLIMATION

We investigate the structure of accretion disks around massive protostar applying steady state models of thin disks. The thin disk equations are solved with proper opacities for dust and gas taking into account the huge temperature variation along the disk. We explore a wide parameter range concerning stellar mass, accretion rate, and viscosity parameter α. The most essential finding is a very high temperature of the inner disk. For e.g., a 10 M ☉ protostar with an accretion rate of ~10–4 M ☉ yr–1, the disk midplane temperature may reach almost 105 K. The disk luminosity in this case is about 104 L ☉ and, thus, potentially higher than that of a massive protostar. We motivate our disk model with similar hot disks around compact stars. We calculate a dust sublimation radius by turbulent disk self-heating of more than 10 AU, a radius, which is 3 times larger than that caused by stellar irradiation. We discuss implications of this result on the flashlight effect and the consequences for the radiation pressure of the central star. In deference to disks around low-mass protostars, our models suggest rather high values for the disk turbulence parameter α ≤ 1. However, disk stability to fragmentation due to thermal effects and gravitational instability would require a lower α value. For α = 0.1, we find stable disks out to 80 AU. Essentially, our model allows us to compare the outer disk to some of the observed massive protostellar disk sources, and from that, extrapolate the disk structure close to the star which is yet impossible to observe.

The full set of gas giant structures: I: On the origin of planetary masses and the planetary initial mass function

Context Current planet search programs are detecting extrasolar planets at a rate of 60 planets per year. These planets show more diverse properties than was expected. Aims We try to get an overview of possible gas giant (proto-) planets for a full range of orbital periods and stellar masses. This allows the prediction of the full range of possible planetary properties which might be discovered in the near future. Methods We calculate the purely hydrostatic structure of the envelopes of proto-planets that are embedded in protoplanetary disks for all conceivable locations: combinations of different planetesimal accretion rates, host star masses, and orbital separations. At each location all hydrostatic equilibrium solutions to the planetary structure equations are determined by variation of core mass and pressure over many orders of magnitude. For each location we analyze the distribution of planetary masses. Results We get a wide spectrum of core–envelope structures. However, practically all calculated proto-planets are in the planetary mass range. Furthermore, the planet masses show a characteristic bimodal, sometimes trimodal, distribution. For the first time, we identify three physical processes that are responsible for the three characteristic planet masses: self-gravity in the Hill sphere, compact objects, and a region of very low adiabatic pressure gradient in the hydrogen equation of state. Using these processes, we can explain the dependence of the characteristic masses on the planet’s location: orbital period, host star mass, and planetesimal accretion rate (luminosity). The characteristic mass caused by the self-gravity effect at close proximity to the host star is typically one Neptune mass, thus producing the so-called hot Neptunes. Conclusions Our results suggest that hot Jupiters with orbital period less than 64 days (the exact location of the boundary depends on stellar type and accretion rate) have quite distinct properties which we expect to be reflected in a different mass distribution of these planets when compared to the “normal” planetary population. We use our theoretical survey to produce an upper mass limit for embedded planets: the maximum embedded equilibrium mass (MEEM). This naturally explains the lack of high mass planets between 3 and 64 days orbital period.

CRYSTALLINE SILICATES AND DUST PROCESSING IN THE PROTOPLANETARY DISKS OF THE TAURUS YOUNG CLUSTER

We characterize the crystalline-silicate content and spatial distribution of small dust grains in a large sample of protoplanetary disks in the Taurus-Auriga young cluster, using the Spitzer Space Telescope mid-IR spectra. In turn we use the results to analyze the evolution of structure and composition of these 1-2 Myr old disks around Solar- and later-type young stars, and test the standard models of dust processing which result in the conversion of originally amorphous dust into minerals. We find strong evidence of evolution of the dust-crystalline mass fraction in parallel with that of the structure of the disks, in the sense that increasing crystalline mass fraction is strongly linked to dust settling to the disk midplane. We also confirm that the crystalline silicates are confined to small radii, r 10 AU. However, we see no significant correlation of crystalline mass fraction with stellar mass or luminosity, stellar-accretion rate, disk mass, or disk/star mass ratio, as would be expected in the standard models of dust processing based upon photoevaporation and condensation close to the central star, accretion-heating-driven annealing at r 1 AU, or spiral-shock heating at r 10 AU, with or without effective large-scale radial mixing mechanisms. Either another grain-crystallizing mechanism dominates over these, or another process must be at work within the disks to erase the correlations they produce. We propose one of each sort that seems to be worth further investigation, namely X-ray heating and annealing of dust grains, and modulation of disk structure by giant-planetary formation and migration.

The ultraviolet luminosity function and star formation rate of the Coma cluster

We present estimates of the GALEX NUV and FUV luminosity functions (LFs) of the Coma cluster, over a total area of ~9 deg^2 (~25 Mpc^2), i.e. from the cluster center to the virial radius. Our analysis represents the widest and deepest UV investigation of a nearby cluster of galaxies made to date. The Coma UV LFs show a faint-end slope steeper than the one observed in the local field. This difference, more evident in NUV, is entirely due to the contribution of massive quiescent systems (e.g. ellipticals, lenticulars and passive spirals), more frequent in high density environments. On the contrary, the shape of the UV LFs for Coma star-forming galaxies does not appear to be significantly different from that of the field, consistently with previous studies of local and high redshift clusters. We demonstrate that such similarity is only a selection effect, not providing any information on the role of the environment on the star formation history of cluster galaxies. By integrating the UV LFs for star-forming galaxies (corrected for the first time for internal dust attenuation), we show that the specific star formation rate of Coma is significantly lower than the integrated SSFR of the field and that Coma-like clusters contribute only <7% of the total SFR density of the local universe. Approximately 2/3 of the whole star-formation in Coma is occurring in galaxies with M_star < 10^10 M_sol. The vast majority of star-forming galaxies has likely just started its first dive into the cluster core and has not yet been affected by the cluster environment. The total stellar mass accretion rate of Coma is ~(0.6-1.8) x 10^12 M_sol Gyr^-1, suggesting that a significant fraction of the population of lenticular and passive spirals observed today in Coma could originate from infalling galaxies accreted between z~1 and z~0.

Near‐Infrared Interferometric, Spectroscopic, and Photometric Monitoring of T Tauri Inner Disks

We present high angular resolution observations with the Keck Interferometer, high dispersion spectroscopic observations with Keck/NIRSPEC, and near-IR photometric observations from PAIRITEL of a sample of 11 solar-type T Tauri stars in 9 systems. We use these observations to probe the circumstellar material within 1 AU of these young stars, measuring the circumstellar-to-stellar flux ratios and angular size scales of the 2.2 micron emission. Our sample spans a range of stellar luminosities and mass accretion rates, allowing investigation of potential correlations between inner disk properties and stellar or accretion properties. We suggest that the mechanism by which the dusty inner disk is truncated may depend on the accretion rate of the source; in objects with low accretion rates, the stellar magnetospheres may truncate the disks, while sublimation may truncate dusty disks around sources with higher accretion rates. We have also included in our sample objects that are known to be highly variable (based on previous photometric and spectroscopic observations), and for several sources, we obtained multiple epochs of spectroscopic and interferometric data, supplemented by near-IR photometric monitoring, to search for inner disk variability. While time-variable veilings and accretion rates are observed in some sources, no strong evidence for inner disk pulsation is found.

The Magnetic Fields of Classical T Tauri Stars

We report new magnetic field measurements for 14 classical T Tauri stars (CTTSs). We combine these data with one previous field determination in order to compare our observed field strengths with the field strengths predicted by magnetospheric accretion models. We use literature data on the stellar mass, radius, rotation period, and disk accretion rate to predict the field strength that should be present on each of our stars according to these magnetospheric accretion models. We show that our measured field values do not correlate with the field strengths predicted by simple magnetospheric accretion theory. We also use our field strength measurements and literature X-ray luminosity data to test a recent relationship expressing X-ray luminosity as a function of surface magnetic flux derived from various solar feature and main-sequence star measurements. We find that the T Tauri stars we have observed have weaker than expected X-ray emission by over an order of magnitude on average using this relationship. We suggest the cause for this is actually a result of the very strong fields on these stars, which decreases the efficiency with which gas motions in the photosphere can tangle magnetic flux tubes in the corona.

Demographics of transition objects

The unusual properties of transition objects (young stars with an optically thin inner disc surrounded by an optically thick outer disc) suggest that significant disc evolution has occured in these systems. We explore the nature of these systems by examining their demographics, specifically their stellar accretion rates (Mdot) and disc masses (Mdisc) compared to those of accreting T Tauri stars of comparable age. We find that transition objects in Taurus occupy a restricted region of the Mdot vs. Mdisc plane. Compared to non-transition single stars in Taurus, they have stellar accretion rates that are typically ~10 times lower at the same disc mass and median disc masses ~4 times larger. These properties are anticipated by several proposed planet formation theories and suggest that the formation of Jovian mass planets may play a significant role in explaining the origin of at least some transition objects. Considering transition objects as a distinct demographic group among accreting T Tauri stars leads to a tighter relationship between disc masses and stellar accretion rates, with a slope between the two quantities that is close to the value of unity expected in simple theories of disc accretion.

The Stellar Mass-Accretion Rate Relation in T Tauri Stars and Brown Dwarfs

Recent observations show a strong correlation between stellar mass and accretion rate in young stellar and substellar objects, with the scaling acc ∝ M holding over more than 4 orders of magnitude in accretion rate. We explore the consequences of this correlation in the context of disk evolution models. We note that such a correlation is not expected to arise from variations in disk angular momentum transport efficiency with stellar mass, and we suggest that it may reflect a systematic trend in disk initial conditions. In this case we find that brown dwarf disks initially have rather larger radii than those around more massive objects. By considering disk evolution, and invoking a simple parameterization for a shutoff in accretion at the end of the disk lifetime, we show that such models predict that the scatter in the stellar mass-accretion rate relationship should increase with increasing stellar mass, in rough agreement with current observations.

A Solution to the Pre-Main-Sequence Accretion Problem

Accretion rates of order 10-8 M☉ yr-1 are observed in young pre-main-sequence (PMS) stars of approximately a solar mass with evidence of circumstellar disks. The accretion rate is significantly lower for PMS stars of smaller mass, approximately proportional to the second power of the stellar mass, accr M2. The traditional view is that the observed accretion is the consequence of the angular momentum transport in isolated circumstellar disks, controlled by disk turbulence or self-gravity. However, these processes are not well understood and the observed accretion, a fundamental aspect of star formation, remains an unsolved problem. In this Letter, we propose the stellar accretion rate is controlled by accretion from the large-scale gas distribution in the parent cloud, not by the isolated disk evolution. Approximating this process as Bondi-Hoyle accretion onto the star-disk system, we obtain accretion rates comparable to the observed ones. We also reproduce the observed dependence of the accretion rate on the stellar mass. These results are based on realistic values of the ambient gas density and velocity, as inferred from numerical simulations of star formation in self-gravitating turbulent clouds.

Mid‐infrared Spectroscopy of Disks around Classical T Tauri Stars

We present the first Spitzer Infrared Spectrograph (IRS; The IRS was a collaborative venture between Cornell University and Ball Aerospace Corporation funded by NASA through the Jet Propulsion Laboratory and the Ames Research Center.) observations of the disks around classical T Tauri stars: spectra in the 5.2-30 micron range of six stars. The spectra are dominated by emission features from amorphous silicate dust, and a continuous component from 5 to 8 microns that in most cases comprises an excess above the photosphere throughout our spectral range. There is considerable variation in the silicate feature/continuum ratio, which implies variations of inclination, disk flaring, and stellar mass accretion rate. In most of our stars, structure in the silicate feature suggests the presence of a crystalline component. In one, CoKu Tau/4, no excess above the photosphere appears at wavelengths shortward of the silicate features, similar to 10 Myr old TW Hya, Hen 3-600, and HR 4796A. This indicates the optically thick inner disk is largely absent. The silicate emission features with peaks at 9.7 and 18 microns indicate small dust grains are present. The extremely low 10-20 micron color temperature of the dust excess, 135 K, indicates these grains are located more than 10 AU from the star. These features are suggestive of gravitational influence by planets or close stellar companions and grain growth in the region within 10 AU of the star, somewhat surprising for a star this young (1 Myr).

Outflows and accretion in a star-disc system with stellar magnetosphere and disc dynamo

The interaction between a protostellar magnetosphere and a surrounding dynamo-active accretion disc is investigated using an axisymmetric mean-field model. In all models investigated, the dynamo-generated magnetic field in the disc arranges itself such that in the corona, the field threading the disc is anti-aligned with the central dipole so that no X-point forms. When the magnetospheric field is strong enough (stellar surface field strength around 2 kG or larger), accretion happens in a recurrent fashion with periods of around 15 to 30 days, which is somewhat longer than the stellar rotation period of around 10 days. In the case of a stellar surface field strength of at least a few 100 G, the star is being spun up by the magnetic torque exerted on the star. The stellar accretion rates are always reduced by the presence of a magnetosphere which tends to divert a much larger fraction of the disc material into the wind. Both, a pressure-driven stellar wind and a disc wind form. In all our models with disc dynamo, the disc wind is structured and driven by magneto-centrifugal as well as pressure forces.

A Direct Measurement of Major Galaxy Mergers atz3

This paper presents direct evidence for hierarchical galaxy assembly out to redshifts z ~ 3. We identify major mergers using the model-independent CAS (concentration, asymmetry, clumpiness) physical morphological system on galaxies detected, and photometrically selected, in the WFPC2 and NICMOS Hubble Deep Field North. We specifically use the asymmetric distributions of rest-frame optical light measured through the asymmetry parameter (A) to determine the fraction of galaxies undergoing major mergers as a function of redshift (z), stellar mass (M*), and absolute magnitude (MB). We find that the fraction of galaxies consistent with undergoing a major merger increases with redshift for all galaxies, but most significantly, at 5?10 ? confidence, for the most luminous and massive systems. The highest merger fractions we find are 40%?50% for galaxies with MB 1010 M? at z > 2.5, e.g., objects identified as Lyman-break galaxies. Using these results, we model the merger fraction evolution in the form fm(A, M*, MB, z) = f0 ? (1 + z). We find mA values ~4?6 for the most luminous and massive galaxies, while lower mass and less luminous galaxies have smaller mA values. We use these merger fractions, combined with merger timescales calculated from N-body simulations, to derive galaxy merger rates to z ~ 3. We also use stellar masses of HDF-N galaxies to determine the mass accretion rate of field galaxies involved in major mergers. We find an average stellar mass accretion rate of G ~ 4 ? 108 M? Gyr-1 per galaxy at z ~ 1 for galaxies with stellar masses M* > 109 M?. This accretion rate changes with redshift as G = 1.6 ? 108(1 + z)0.99?0.32 M? Gyr-1 per galaxy. We also find that the fraction of stellar mass density in galaxies involved in major mergers increases with redshift, with a peak mass fraction ~0.5 for the brightest, MB 1010 M?, systems near z ~ 2.5. By comparing merger fractions predicted in cold dark matter semianalytic models with our results we find a reasonably good agreement for the largest and brightest systems, although we find more low-mass galaxy mergers at lower redshifts than what these models predict.

New Tests of Magnetospheric Accretion in T Tauri Stars

We examine 3 analytic theories of magnetospheric accretion onto classical T Tauri stars under the assumption that the magnetic field strength does not vary appreciably from star to star. From these investigations we derive predicted relationships among the stellar mass, radius, rotation period, and disk accretion rate. Data from 5 studies of the accretion parameters of CTTSs are used to test the predicted correlations. We generally find that the data do not display the predicted correlations except for that predicted by the model of Shu et al. as detailed by Ostriker and Shu and extended here to include non-dipole field topologies. Their identification of the trapped flux as an important quantity in the model appears to be critical for reconciling the observed data to the theory. While the data do generally support the extended Ostriker and Shu predictions, only one of the two studies for which the requisite data exist show the highest correlation when considering all the relevant parameters. This suggests great care must be taken when trying to use existing observations to test the theory.

Transonic Magnetic Slim Accretion Disks and Kilohertz Quasi‐periodic Oscillations in Low‐Mass X‐Ray Binaries

The inner regions of accretion disks of weakly magnetized neutron stars are affected by general relativistic gravity and stellar magnetic fields. Even for field strengths sufficiently small so that there is no well-defined magnetosphere surrounding the neutron star, there is still a region in the disk where magnetic field stress plays an important dynamical role. We construct magnetic slim disk models appropriate for neutron stars in low-mass X-ray binaries (LMXBs), which incorporate the effects of both magnetic fields and general relativity (GR). The magnetic field-disk interaction is treated in a phenomenological manner, allowing for both closed- and open-field configurations. We show that even for surface magnetic fields as weak as 107-108 G, the sonic point of the accretion flow can be significantly modified from the pure GR value (near rGR = 6GM/c2 for slowly rotating neutron stars). We derive an analytical expression for the sonic radius in the limit of small disk viscosity and pressure. We show that the sonic radius mainly depends on the stellar surface field strength B0 and mass accretion rate ${u{M}{705F}}$ --> through the ratio b -->2 βB -->20/${u{M}{705F}}$ -->, where β |B/Bz| measures the azimuthal pitch angle of the magnetic field threading the disk. The sonic radius thus obtained approaches the usual Alfven radius for high b2 (for which a genuine magnetosphere is expected to form) and asymptotes to 6GM/c2 as b2 → 0. We therefore suggest that for neutron stars in LMXBs, the distinction between the disk sonic radius and the magnetosphere radius may not exist; there is only one generalized sonic radius, which is determined by both the GR effect and the magnetic effect. We apply our theoretical results to the kilohertz (kHz) quasi-periodic oscillations (QPOs) observed in the X-ray fluxes of LMXBs. If these QPOs are associated with the orbital frequency at the inner radius of the disk, then the QPO frequencies and their correlation with mass accretion rate can provide useful diagnostics on the (highly uncertain) nature of the magnetic field-disk interactions. In particular, a tight upper limit to the surface magnetic field B0 can be obtained, i.e., B0 3 × 10 -->7(${u{M}{705F}}$ --> -->17/β) -->1/2 G, where ${u{M}{705F}}$ --> -->17=${u{M}{705F}}$ -->/(10 -->17 g s-1), in order to produce kHz orbital frequency at the sonic radius. Current observational data may suggest that the magnetic fields in LMXBs have complex topology.

Layered Accretion in T Tauri Disks

We put forward a model for accretion disks around T Tauri stars. The model assumes that angular momentum transport is driven by magnetic fields and can occur only in those parts of the disk that are sufficiently ionized that the gas can couple to the magnetic field. These regions lie at R ≲ 0.1 AU, where collisional ionization is effective, and at R ≲ 0.1 AU in a layer of thickness ≍ 100 g cm<SUP>-2</SUP> at the surface of the disk where cosmic-ray ionization is effective. <P />The model predicts that the stellar accretion rate is about 10<SUP>-8</SUP> M<SUB>sun</SUB> yr<SUP>-1</SUP>, independent of the rate of infall onto the disk. Matter that is not accreted onto the star accumulates in the inner few AU of the disk at a rate of about 10<SUP>-3</SUP> M<SUB>sun</SUB> in 10<SUP>4</SUP> yr. Given this buildup it is unlikely that accretion is steady. The effective temperature profile is T<SUB>e</SUB> ∼ r<SUP>-1/2</SUP> outside of 0.1 AU, which differs from the canonical T<SUB>e</SUB> ∼ r<SUP>-3/4</SUP>. We calculate the expected spectral energy distribution for the disk and show that this temperature profile produces an infrared excess. Finally, we discuss some of the leading uncertainties in the theory.

Bolometric temperature and young stars in the Taurus and Ophiuchus complexes

We calculated bolometric temperature (T(sub bol)) and luminosity (L(sub bol)) for 128 young stellar objects (YSOs) in Taurus, 74 in the Ophiuchus 'core', and 33 in the Ophiuchus 'off-core' region. We have constructed the bolometric luminosity-temperature (BLT) diagram, the log-log plot of L(sub bol) versus T(sub bol), for the three samples. T(sub bol) is defined as the temperature of a blackbody having the same frequency as the observed continuum spectrum. It measures the redness (or coldness) of an astronomical source. The BLT diagram is analogous to the H-R diagram and allows for a direct and quantitative comparison of YSOs at a wide variety of evolutionary states, ranging from the most deeply embedded stars to T Tauri stars nearly on the main sequence. We found (1) T(sub bol) increases monotonically from embedded sources (approximately 60-500 K) to classical T Tauri stars (approximately 1000-3000 K) to weak-line T Tauri stars (approximately 2000-5000 K); (2) T(sub bol) correlates reasonably well with the age inferred from the evolutionary models of pre-main-sequence stars and protostars for embedded 'protostars' and weak-line T Tauri stars. There is no significant correlation for the classical T Tauri stars. These results can be understood in terms of dissipation of circumstellar dust envelope and disk during the early stages of stellar evolution. Sources in the three regions have different distributions in the BLT diagram. The Ophiuchus core has the highest fraction of cold sources among the three regions. These cold sources are also more luminous than the YSOs in the other regions. The Ophiuchus off-core sample is dominated by the more evolved pre-main-sequence stars. The Taurus sources have distributions intermediate in L(sub bol), T(sub bol), and age between the Ophiuchus core and off-core distributions. These may suggest differences in the star formation history, and possibly in the stellar masses and mass accretion rates in these star-forming regions.

Collapse and fragmentation of molecular cloud cores. 2: Collapse induced by stellar shock waves

The standard scenario for low-mass star formation involves 'inside-out' collapse of a dense molecular cloud core following loss of magnetic field support through ambipolar diffusion. However, isotopic anomalies in presolar grains and meteoritical inclusions imply that the collapse of the presolar cloud may have been triggered by a stellar shock wave. This paper explores 'outside-in' collapse, that is, protostellar collapse initiated directly by the compression of quiescent dense cloud cores impacted by relatively slow stellar shock waves. A second-order accurate, gravitational hydrodynamics code has been used to study both the spherically symmetrical and three-dimensional evolution of initially centrally condensed, isothermal, self-gravitating, solar-mass cloud cores that are struck by stellar shock waves with velocities up to 25 km/s and postshock temperatures of 10 to 10,000 K. The models show that such mild shock waves do not completely shred and destroy the cloud, and that the dynamical ram pressure can compress the cloud to the verge of self-gravitational collapse. However, compression caused by a high postshock temperature is a considerably more effective means of inducing collapse. Shock-induced collapse produces high initial mass accretion rates (greater than 10(exp -4) solar mass/yr in a solar-mass cloud) that decline rapidly to much lower values, depending on the presence (approximately 10(exp -6) solar mass/yr) or absence (approximately 10(exp -8) to 10(exp -7) solar mass/yr) of an infinite reservoir of mass. Stellar mass accretion rates approximately 10(exp -7) solar mass/yr have been previously inferred from the luminosities of T Tauri stars; balanced mass accretion (stellar rate = envelope rate) at approximately 10(exp -7) solar mass/yr could then be possible if accretion occurs from a finite mass reservoir. Fluid tracers are used to determine what fraction of the stellar shock material is incorporated into the resulting protostellar object and disk; roughly half the impinging material is injected into the collapsing cloud core when there is a high postshock temperature. The models are consistent with a scenario where an AGB star wind triggered the collapse of the presolar cloud while injecting about 0.01 solar mass of matter derived from the AGB star envelope, as has been separately inferred on the basis of nucleosynthesis calculations.

Irradiation of accretion disks around young objects. I - Near-infrared CO bands

view Abstract Citations (223) References (44) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Irradiation of Accretion Disks around Young Objects. I. Near-Infrared CO Bands Calvet, Nuria ; Patino, Alberto ; Magris, Gladis C. ; D'Alessio, Paola Abstract The effect of irradiation by the central star on the atmospheres of optically thick physically thin accretion disks around young objects are approximated, assuming radiative equilibrium. Irradiation is found to increase the temperature in the atmosphere of a viscous accretion disk relative to the case when only the viscous flux goes through it. Its effect on the near-infrared spectrum of the star-disk configuration is to decrease the strength of the absorption in the CO bands or turn them into emission, depending on the irradiation rate and on the mass accretion rate. As the stellar effect temperature increases for a given mass accretion rate, the disk surface temperature increases and the CO bands turn into emission. Observed spectra of young objects such as T Tauri stars, FU Ori subjects, and Herbig Be/Ae stars are in qualitative agreement with those predicted using the corresponding stellar parameters and disk mass accretion rates. It is inferred that the near-infrared CO bands can be used as indicators of the rate of mass accretion in the disk around young objects. Publication: The Astrophysical Journal Pub Date: October 1991 DOI: 10.1086/170618 Bibcode: 1991ApJ...380..617C Keywords: Accretion Disks; Carbon Monoxide; Near Infrared Radiation; Radiative Transfer; Stellar Atmospheres; A Stars; B Stars; Computational Astrophysics; Stellar Mass Accretion; Stellar Temperature; T Tauri Stars; Astrophysics; RADIATIVE TRANSFER; STARS: ACCRETION; STARS: PRE--MAIN-SEQUENCE full text sources ADS | data products SIMBAD (6) Related Materials (1) Part 2: 1992RMxAA..24...27C